The present invention relates to the field of devices for generating shock waves for medical applications, and in particular to a device for generating shock waves for medical applications that includes electrodes that are located in a liquid medium and driven to generate an electrical breakdown between the electrodes, where the medium has a structural viscosity with a thixotropic index greater than one.
Shock waves are increasingly employed in medicine, for example for breaking up concretions in the body, for stimulating bone formation, for treating orthopedic diseases, and for combating pain. To generate shock waves, devices are often used in which two electrodes are arranged a certain distance apart in a liquid medium. A high electric voltage is applied to the electrodes until an electrical breakdown results within the liquid medium. The heat developed from the electrical breakdown causes the liquid medium to vaporize, thereby creating the pressure wave. U.S. Pat. Nos. 6,080,119 and 6,113,560 both disclose devices for generating shock waves.
The action of triggering the spark discharge causing the electrical breakdown depends on the high voltage applied and the distance between the electrodes. To ensure reliable triggering of the spark discharge even at the critical distance between the electrodes, conducting, semiconducting, or polarizable particles in powder form may be added to the liquid aqueous medium. See for example U.S. Pat. No. 6,113,560 assigned to assignee of the present invention. However, the gas formed during the spark discharge may impair the formation of subsequent spark discharges, and specifically impede the propagation of the shock wave. A known remedy disclosed in published German patent application DE 197 18 451 A1 is to add a powdered catalyst to the liquid medium that reduces the gas formation and promotes the recombination of the gases formed. Both the particles promoting the spark discharge and the particles of the catalyst are added in powdered form and suspended in the liquid medium.
To ensure that the particles have the desired effect, they must be suspended specifically in the vicinity of the electrodes. Counteracting this suspension is gravity which, over time, causes the particles increasingly to settle on the bottom of the container holding the liquid medium. A lowering of the concentration of the particles promoting discharge in the vicinity of the electrodes may impair or completely impede the triggering of the spark discharge. A lowering of the concentration of the suspended catalyst particles in the vicinity of the electrodes may also result in premature formation of gas, which will delay the formation of the spark discharge and reduce the generation of pressure.
To counteract the undesirable settling of the suspended particles, the volume of liquid surrounding the electrodes may be restricted by adding a hood. Although the particles still settle due to gravity, they are agitated by each discharge and again are dispersed in suspension. Several discharges may be required, however, until the optimal suspension for operating the shock-wave source is reached. In addition, the initial voltage employed must be higher in order to achieve this first electrical breakdown. Only after the particles promoting the breakdown are sufficiently suspended can the voltage be returned to its normal operating level.
Another approach is to store the powder particles in an ancillary container, which is introduced into the volume of the liquid medium. Upon each discharge, particles are released from this container by the resulting pressure wave and enter the suspension.
Both approaches have the disadvantage that additional components (e.g., hood, or a storage container) are required that must be introduced into the volume of the liquid medium. In addition to the extra design and materials expense, these components disturb the propagation of the generated shock wave. Additionally, the shock-wave source does not immediately achieve the optimum properties at start-up.
Therefore, there is a need for a device for generating shock waves in which the particles suspended in the liquid medium may be retained in the vicinity of the electrodes without the extra design and materials expense and without impeding the generation of the shock waves.
Briefly, according to an aspect of the present invention, a device for generating shock waves for medical uses by a high voltage electrical discharge, includes a pair of electrical spark charge electrodes and a liquid medium disposed between the electrodes. A catalyst is dispersed in the liquid medium. An inorganic silicon compound is also added to the liquid medium, wherein the liquid medium containing the catalyst and the inorganic silicon compound includes a structural viscosity with a thixotropic index greater than one.
According to another aspect of the invention, a liquid medium for use in a device for generating shock waves for medical uses by applying a voltage between electrodes located within the liquid medium, comprises water and a catalyst dispersed in the water. An inorganic silicon compound is added to the water, wherein the liquid medium includes a structural viscosity with a thixotropic index greater than one.
The structural viscosity of the liquid medium is obtained by adding an inorganic silicon compound, such as silicic acid. A preferred additive is a highly dispersed silicic acid.
The highly dispersed silicic acid does not significantly affect the electrical properties of the liquid medium. The conductivity of the liquid medium, the latency period required for the formation of the electrical breakdown, and the quantity of the charge flowing from the breakdown all remain unaffected by the added silicic acid. In addition, silicic acid is heat-resistant and chemically stablexe2x80x94with the result that the properties of the structural viscosity are not destroyed by the plasma created during breakdown. The metal alloy of the electrodes is also not corroded by the added silicic acid.
These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings.